Method of polymerizing deionized bis-β-hydroxyethyl terephthalate

ABSTRACT

A method of producing a polyester from high-purity bis-β-hydroxyethyl terephthalate as a raw material containing cations and anions in trace amounts. Polyester production raw materials including bis-β-hydroxyethyl terephthalate containing cations and anions in a total weight of not more than 50 ppm, ethylene glycol and antimony trioxide and/or germanium dioxide as a polymerization catalyst are heated without substantially distilling off ethylene glycol and then heated under reduced pressure to be polycondensed while ethylene glycol is distilled off.

FIELD OF THE INVENTION

The present invention relates to a polyester production method and topolyester production raw materials. More specifically, it relates to amethod of producing a polyester by using deionized high-puritybis-β-hydroxyethyl terephthalate containing cations and anions in atotal weight of not more than 50 ppm as part of a raw material for theproduction of a polyester.

PRIOR ART

In the knowledge that a polyester, especially a polyester essentiallycomposed of polyethylene terephthalate exhibits performance well adaptedto various application purposes, the polyester has been widely used infibers, films and resins. In recent years, it has been used in a widervariety of fields. Various industrial methods for producing a polyesterhave been proposed. Out of these, a method in which polyethyleneterephthalate is obtained by polycondensing a reaction product obtainedby directly esterifying terephthalic acid and ethylene glycol orcarrying out an ester exchange reaction between a lower alkyl ester ofterephthalic acid, for example, dimethyl terephthalate and ethyleneglycol at a high temperature under high vacuum is widely put topractical use currently.

As the polyester is used for various purposes as described above, higherquality is required of the polyester for the purposes in terms ofvarious items. Since the polyester is basically produced by highlypolymerizing an esterified product or ester exchange reaction product ata high temperature under high vacuum in the presence of various metalcompounds as a polymerization catalyst by maintaining it in a moltenstate for a long time, it is not easy to satisfy all the requirements.Under the situation, it is important not only to obtain higher qualitypolyester raw materials but also to bring out the function of apolymerization catalyst to the full extent. The present inventors havefound that a polyester raw material from which cations and anionsgenerally existent therein have been removed satisfies requirements forhigher quality and proposed separately. The present inventors conductedfurther studies on a polymerization method when the specific polyesterraw material is used and accomplished the present invention.

SUMMARY OF THE INVENTION

That is, it is an object of the present invention to provide a method ofproducing a polyester from high-purity bis-β-hydroxyethyl terephthalatecontaining cations and anions in trace amounts.

It is another object of the present invention to provide a novel methodof producing a polyester from the above high-purity bis-β-hydroxyethylterephthalate based on a surprising fact discovered by the presentinventors that polycondensation does not proceed at a substantial speedeven when a polymerization catalyst such as antimony trioxide is addedto the above high-purity bis-β-hydroxyethyl terephthalate and heatedunder reduced pressure.

It is still another object of the present invention to provide anindustrially advantageous method of producing a copolymerizationpolyester containing a polyethylene terephthalate or ethyleneterephthalate polymerization unit from the above high-puritybis-β-hydroxyethyl terephthalate.

It is a further object of the present invention to provide a method ofproducing a polyester by using the above high-purity bis-β-hydroxyethylterephthalate as at least part of a raw material for the production of apolyester.

Other objects and advantages of the present invention will becomeapparent from the following description.

According to the present invention, the above objects and advantages ofthe present invention are attained by a method for producing a polyestercomprising the steps of:

(1) providing polyester production raw materials includingbis-β-hydroxyethyl terephthalate containing cations and anions in atotal weight of not more than 50 ppm, ethylene glycol and at least onepolymerization catalyst selected from the group consisting of antimonytrioxide and germanium dioxide;

(2) heating the polyester production raw materials without distillingoff ethylene glycol; and

(3) polycondensing the raw materials by heating under reduced pressurewhile distilling off ethylene glycol.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The bis-β-hydroxyethyl terephthalate prepared in the step (1) of thepresent invention contains cations and anions in a total weight of notmore than 50 ppm. The bis-β-hydroxyethyl terephthalate used in thepresent invention embraces not only what contains both anions andcations but also what contains only anions or cations if the totalweight of cations and anions is not more than 50 ppm. Thebis-β-hydroxyethyl terephthalate used in the present invention may becalled “deionized bis-β-hydroxyethyl terephthalate”. The cations as usedherein are those of (I) Na, Mg, Ca, Fe, Co, Zn, Ti, Sn, Sb, Ge and P,and the anions are those of (II) halogen, NO₂, NO₃, PO₄ and SO₄.

According to the results of studies conducted by the present inventors,a polyester obtained from the deionized bis-β-hydroxyethyl terephthalatehas high quality.

According to the results of studies conducted by the present inventors,it has been found that it is practical to substantially decationize anddeanionize a solution containing ethylene glycol as the main solvent andbis-β-hydroxyethyl terephthalate as the main solute using an ionexchanger, especially an ion exchange resin, in order to preparedeionized bis-β-hydroxyethyl terephthalate used in the presentinvention. An example of the ion exchange resin for decationizationsuitable for the embodiment above is an Amberlite cation exchange resin(of Organo Co., Ltd.) and an example of the ion exchange resin fordeanionization suitable for the embodiment above is an Amberlite anionexchange resin (of Organo Co., Ltd.). The step in which this ionexchange resin is used can be carried out by using a conventionallyknown method. Especially when decationization and deanionizationoperations are to be carried out, it is preferred to select atemperature at which bis-β-hydroxyethyl terephthalate does not separateout in a solution containing ethylene glycol as the main solvent andbis-β-hydroxyethyl terephthalate as the main solute and the ion exchangeresin can be stably used and the concentration of bis-β-hydroxyethylterephthalate in the solution. A decolorization step can be added beforeor after this step, whereby the total content of cations and anions canbe reduced to 50 ppm or less and bis-β-hydroxyethyl terephthalate havingexcellent whiteness can be obtained advantageously. It is preferred tocarry out the decolorization step by passing through a decolorizingmaterial such as activated carbon in the same manner as thedecationization and deanionization operations.

According to the results of studies conducted by the present inventors,it has been found for the first time that when the deionizedbis-β-hydroxyethyl terephthalate is used as at least part of a rawmaterial for the production of a polyester, particularly the thermalstability of the obtained polyester when it is to be put to practicaluse as a fiber, film or molded product such as a bottle becomes high,making molding easy. Further, when polyester molded products are to bedepolymerized, substantially restored to the stage of bis-β-hydroxyethylterephthalate and used as a raw material for the production of apolyester, a high-quality polyester can be practically produced by usingthe above deionized bis-β-hydroxyethyl terephthalate in the method ofthe present invention. Stated more specifically, a depolymerized productobtained by depolymerizing polyester molded products using ethyleneglycol can be obtained as a solution containing ethylene glycol as themain solvent as described above. Deionized bis-β-hydroxyethylterephthalate specified by the present invention can be used again as araw material for the production of a high-quality polyester bysubjecting the solution to the step of removing cations and anions orthe decolorizing step as required directly or after the concentration ofthe solution has been adjusted to an appropriate level. Even when apolyester molded product to be depolymerized is a commercial product ormixed with other materials or contains foreign matter such as dust inthis case, depolymerization can be carried out smoothly by applying theforeign matter removing step such as sorting or filtration as required.For example, when a polyester is in the form of a fibrous commercialproduct, it may be mixed with a different type of fibers or contain aninorganic material such as titanium oxide used in the polyester, when apolyester is in the form of a film, it may be mixed with a differenttype of film material or contain various lubricants used in thepolyester, when a polyester is in the form of another molded productsuch as a bottle, it may be ground and mixed with a different type ofmaterial such as polyethylene used in a cover portion or bottom portion,and paper or plastic used as a label. All the above situations arerather common. According to the results of studies conducted by thepresent inventors, high-quality bis-β-hydroxyethyl terephthalate can beeasily obtained by applying a conventionally known technique such asliquid-liquid separation or solid-liquid separation and then carryingout depolymerization, deionization and decolorization steps.

Further, high-quality bis-β-hydroxyethyl terephthalate having a totalweight of purified specific ions of 50 ppm or less and used in thepresent invention can be obtained by subjecting the obtained solutioncontaining the substantially decationized and deanionizedbis-β-hydroxyethyl terephthalate to a molecular distillation step.

The expression “molecular distillation steps” as used herein meansnon-equilibrium distillation that one-way movement of molecules ofevaporated bis-β-hydroxyethyl terephthalate from an evaporation plane toa condensation plane occurs without returning to the evaporation planeagain and not boiling-point distillation at a distillation temperatureand pressure, that is, equilibrium distillation.

The bis-β-hydroxyethyl terephthalate having a total weight of ionsspecified by the present invention and generally existent of more than50 ppm and the deionized bis-β-hydroxyethyl terephthalate used in thepresent invention behave in completely different ways when a polyesteris to be produced. For example, the bis-β-hydroxyethyl terephthalatehaving a total weight of generally existent ions of more than 50 ppm isused as a raw material, antimony trioxide or germanium dioxide in theform of a powdery solid is added to molten bis-β-hydroxyethylterephthalate, whereby a polymerization reaction proceeds relativelyswiftly at a high temperature under high vacuum. When the deionizedbis-β-hydroxyethyl terephthalate used in the present invention is usedas a raw material, polymerization does not proceed at a substantialspeed and a polyester obtained from long term of a polymerizationreaction at a high temperature under high vacuum is colored yellow brownto a visible extent.

In the step (2) of the present invention, the above polyester productionraw material is heated without distilling off ethylene glycolsubstantially.

Heating which is carried out without distilling off ethylene glycolsubstantially is preferably carried out at 150 to 200° C. for 30 to 90minutes. Heating may be carried out at normal pressure or reducedpressure but the reflux of ethylene glycol is preferably carried out sothat ethylene glycol is not substantially distilled out. An oligomer ofthe bis-β-hydroxyethyl terephthalate has an average polymerizationdegree of preferably 2 or more, more preferably 3 or more, particularlypreferably 5 or more. The time for obtaining the oligomer of thebis-β-hydroxyethyl terephthalate which depends on the temperaturecondition is generally 60 minutes or less, preferably 40 minutes orless, more preferably 30 minutes or less. The temperature is preferably270° C. or less, more preferably 260° C. or less, much more preferably250° C. or less. The pressure may be vacuum but preferably normalpressure or slightly reduced pressure to reduce spray of the oligomer.

As described above, it has been found by the present inventors thatpolymerization does not substantially proceed even by adding apolymerization catalyst such as antimony trioxide to the deionizedbis-β-hydroxyethyl terephthalate and heating under reduced pressure. Thepolyester production raw materials in the present invention includeethylene glycol as an essential ingredient in addition to the deionizedbis-β-hydroxyethyl terephthalate and a polymerization catalyst.

Antimony trioxide, germanium dioxide or a combination thereof is used asthe polymerization catalyst.

As for the amount of the polymerization catalyst in the presentinvention, antimony trioxide is used in an amount of preferably 450 ppmor less, more preferably 350 ppm or less, particularly preferably 300ppm or less in terms of antimony based on 1 part by weight of apolyester obtained by polymerization. Germanium dioxide is used in anamount of 200 ppm or less, preferably 150 ppm or less, more preferably120 ppm or less in terms of germanium based on the same standard.Antimony trioxide and germanium dioxide may be used in combinationwithin the above respective ranges. Further, another conventionallyknown polymerization catalyst such as a titanium compound exemplified bytitanium tetrabutoxide may be used in combination within aconventionally known range.

The polymerization catalyst which is acquired as a powdery solid may beused directly or after it is dispersed in ethylene glycol at normaltemperature or after it is dispersed in ethylene glycol and heated. Outof these, it is preferably used after it is dispersed in ethylene glycoland heated. The heating time is preferably 10 minutes or more, morepreferably 20 minutes or more, particularly preferably 30 minutes ormore. The heating temperature is preferably 260° C. or less, morepreferably 220° C. or less, particularly preferably 200° C. or less.These steps may be carried out at normal pressure, increased pressure orreduced pressure unless ethylene glycol is distilled off to the outsideof the system instantaneously. The fact that a high-quality polyestercan be effectively obtained from the deionized bis-β-hydroxyethylterephthalate by heating the polymerization catalyst in ethylene glycolhas been found through studies conducted by the present inventor for thefirst time and a really surprising result. The reason for this isunknown but it is considered that the alcoholation of antimony and/orgermanium is promoted to exhibit the function of the polymerizationcatalyst swiftly.

The amount of ethylene glycol out of the polyester production rawmaterials is preferably 2 wt % or more, more preferably 4 wt % or more,particularly preferably 5 wt % or more based on the bis-β-hydroxyethylterephthalate. The upper limit is preferably 10 wt %.

According to the method of the present invention, a high-qualitypolyester can be produced by using the deionized bis-β-hydroxyethylterephthalate as all of its raw material or by using the deionizedbis-β-hydroxyethyl terephthalate as part of the raw material. Forexample, a high-quality polyester can be produced by usingconventionally known high-quality bis-β-hydroxyethyl terephthalatetogether with the above deionized bis-β-hydroxyethyl terephthalate ormixing the above deionized bis-β-hydroxyethyl terephthalate withhigh-purity terephthalic acid.

That is, a preferred compound which may be further included in thepolyester production raw materials is terephthalic acid, isophthalicacid or 1,4-cyclohexanedimethanol. Terephthalic acid is used in anamount of preferably 0.05 to 20 mols, more preferably 0.1 to 15 mols,particularly preferably 0.15 to 10 mols based on 1 mol ofbis-β-hydroxyethyl terephthalate. The polyester production raw materialsincluding terephthalic acid in the present invention can be prepared byadding the deionized bis-β-hydroxyethyl terephthalate to a mixture ofraw materials consisting of terephthalic acid and ethylene glycol in adirect polymerization method for producing a polyester from terephthalicacid and ethylene glycol by direct esterification.

Isophthalic acid and/or 1,4-cyclohexanedimethanol are/is used in anamount of preferably 0.05 to 50 mols, more preferably 0.1 to 40 mols,particularly preferably 0.15 to 35 mols based on 1 mol ofbis-β-hydroxyethyl terephthalate. In this case, the obtained polyesteris a copolyester containing a recurring unit such as ethyleneisophthalate or 1,4-cyclohexane dimethylene terephthalate in addition toethylene terephthalate.

The polyester to be produced in the present invention includes acopolymer containing a third component such as isophthalic acid or1,4-cyclohexanedimethanol as described above.

As for examples of the third component to be copolymerized, dicarboxylicacids include aromatic dicarboxylic acids other than isophthalic acid,such as diphenyldicarboxylic acid, diphenylsulfone dicarboxylic acid,diphenyl ether dicarboxylic acid, naphthalenedicarboxylic acid,diphenoxyethane dicarboxylic acid and sodium sulfoisophthalic acid,aliphatic dicarboxylic acids such as sebacic acid and adipic acid,alicyclic dicarboxylic acids such as hexahydroterephthalic acid. Diolsother than 1,4-cyclohexanedimethanol include diethylene glycol,trimethylene glycol, tetramethylene glycol, hexamethylene glycol,bis-β-hydroxyethyl bisphenol A, bis-β-hydroxyethoxydiphenyl sulfone,bis-β-hydroxyethoxydiphenyl ether, polyethylene glycol and the like.Hydroxycarboxylic acids such as p-hydroxyethoxyphenylcarboxylic acid mayalso be used. Further, a polyfunctional compound having a functionalityof 3 or more and/or a monofunctional compound may be used in combinationwithin the range of the present invention. Examples of thepolyfunctional compound having a functionality of 3 or more includetrimesic acid, glycerin, pentaerythritol and the like, and examples ofthe monofunctional compound include diphenylmonocarboxylic acid,diphenyl ether monocarboxylic acid, phenoxypolyethylene glycol and thelike. These copolymerizable components may be used as an ester orfunctional derivative.

The copolymerizable component is desirably mixed into the polyesterproduction raw materials. The copolymerizable component is used in anamount of preferably 20 mol % or less, more preferably 15 mol % or less,much more preferably 10 mol % or less based on the total number of molsof an acid component and a diol component.

Since the polyester is basically highly polymerized in the presence of ametal compound such as a polymerization catalyst at a high temperatureunder high vacuum by maintaining in a molten state for a long time, astabilizer for the polyester is preferably used in combination.

Even when the deionized high-quality bis-β-hydroxyethyl terephthalatespecified in the present invention is used as at least part of a rawmaterial for the production of a polyester, a higher-quality polyestercan be produced by using a stabilizer. Any known phosphorus compound maybe used as the stabilizer. However, in order to produce a higher-qualitypolyester, a known phosphorus compound is preferably existent to preventthe deterioration of bis-β-hydroxyethyl terephthalate in advance. Thetemperature for mixing the phosphorus compound is preferably 200° C. orless, more preferably 150° C. or less, much more preferably 135° C. orless, particularly preferably a temperature lower than the melting pointof the bis-β-hydroxyethyl terephthalate. Known phosphorus compoundsinclude phosphoric acid, phosphorous acid, tributyl phosphate and thelike. The amount of the phosphorus compound is preferably 50 ppm orless, more preferably 40 ppm or less, much more preferably 30 ppm orless in terms of phosphorus based on 1 part by weight of the polyesterobtained by polymerization. The stabilizer is not limited to one knownphosphorus compound but a plurality of phosphorus compounds may be usedin combination.

In the present invention, other conventionally known catalysts andadditives may be used in limits that do not impair effects of thepresent invention. For example, anatase type titanium dioxide may beadded as a flatting agent.

In the above step (3), the reaction product formed in the step (2) isheated under reduced pressure to carry out polycondensation whiledistilling off ethylene glycol. The step (3) is preferably carried outby distilling off excess ethylene glycol relatively gently to obtain anoligomer of bis-β-hydroxyethyl terephthalate and by preventing thebis-β-hydroxyethyl terephthalate from being sprayed in large quantitiesunder high vacuum to efficiently carry out a polycondensation reaction.

The step (3) can be carried out under a high vacuum of 5 to 0.1 mmHg ata temperature of 260 to 300° C. The polycondensation time which dependson the polymerization degree of the obtained polyester is 0.5 to 6hours, for example.

The polyester obtained by the method of the present invention isadvantageously used for the production of molded products such asfibers, films and bottles.

In the present invention, the contents of anions and cations in thebis-β-hydroxyethyl terephthalate is obtained and defined as follows.

Content of Cations

This was carried out by inductive coupling plasma light emissionspectral analysis (ICP-AES).

(1) pretreatment

A sample containing ethylene glycol and bis-β-hydroxyethyl terephthalatewas heated at about 80° C. to prepare a uniform solution, 11 g of thissolution was accurately weighed, transferred to a conical beaker andheated at about 220° C. to remove ethylene glycol, 20 ml of sulfuricacid was added and heated, and nitric acid was added in an amount of 1ml each time until nitrogen oxide was not formed any longer to decomposean organic material.

(2) measurement

This sample was cooled to room temperature, 5 ml of hydrochloric acidwas added, and the weight of cations contained in a solution whosequantity was adjusted to 100 ml with ultra pure water was measured byICS-AES. The cations to be measured are those of Na, Mg, Ca, Fe, Co, Zn,Ti, Sn, Sb, Ge and P. The total weight of these is taken as the contentof cations.

(3) measuring device

The amount of water was measured by the Karl Fischer's moisture meter ofKyoto Denshi Kogyo Co., Ltd.

ICS-AES was carried out by the ICAP-575 of Nippon Jarrel Ash Co., Ltd.

Content of Anions

This was carried out by ion chromatography.

(1) pretreatment

A sample containing ethylene glycol and bis-β-hydroxyethyl terephthalatewas heated at about 80° C. to prepare a uniform solution, about 11 g ofthis solution was accurately weighed, and 100 ml of ultra pure water wasadded to this solution and shaken to extract ion components into a waterphase.

(2) measurement

The extracted water phase was filtered with a 0.2 μm-mesh filter tomeasure the weight of anions by ion chromatography. The anions to bemeasured are those of Cl, Br, F, NO₂, NO₃, PO₄ and SO₄, and the totalweight of the anions is taken as the content of anions.

(3) measuring device

The ion chromatograph was the IC-7000S of Yokogawa Denki Co., Ltd. TheIonPacAS4A-SC of Dionecs Co., Ltd. was used as a measurement column tomeasure NO₂, NO₃, PO₄, Cl and Br, and the IonPacAS12A of Dionecs Co.,Ltd. was used to measure F.

The following examples are provided for the purpose of furtherillustrating the present invention but are in no way to be taken aslimiting.

EXAMPLES Example 1

(1) 53 kg of ground flakes of a used PET bottle (made from apolyethylene terephthalate resin) and 298 kg of ethylene glycol werecharged into a 1 m³ autoclave equipped with a stirrer, 0.27 kg of sodiummethylate was added as a known ester exchange catalyst to carry outdepolymerization at 200° C. and normal pressure for 4 hours so as toprepare a solution containing ethylene glycol as the main solvent andbis-β-hydroxyethyl terephthalate as the main solute, and the solutionwas wholly decolorized with active carbon by reducing the temperature to55° C. to obtain 350 kg of a raw solution. The total weight of cationsin the concentrated solute of the raw solution was 2,080 ppm and thetotal weight of anions was 22 ppm. 150 kg of this raw solution wasdecationized with a cation exchange resin (Amberlite IR120-B of OrganoCo., Ltd.) and deanionized with an anion exchange resin (AmberliteIRA-400 of Organo Co., Ltd.) at 55° C. After deionization, the totalweight of cations in the concentrated solute of the solution was 9.4 ppmand the total weight of anions was 0 ppm. The decationized anddeanionized solution was charged into a 500 liter autoclave equippedwith a stirrer and vacuum generator, ethylene glycol was distilled offat 135° C. and 10,670 Pa (80 mmHg) until the weight of residual ethyleneglycol in the solution became 20%, and the solution was concentrated bya vacuum thin film evaporator having a heat transfer area of 0.5 m² at150° C. and 200 Pa (1.5 mmHg) until the content of a material having aboiling point lower than the boiling point of bis-β-hydroxyethylterephthalate became 5.0 wt % to obtain 31.6 kg of a compositioncontaining crude bis-β-hydroxyethyl terephthalate. 31.6 kg of thecomposition containing crude bis-β-hydroxyethyl terephthalate wasmolecular distilled by a molecular distiller having a heat transfer areaof 0.5 m² at 200° C. and 24 Pa (0.18 mmHg) for 75 minutes to obtain 29.4kg of purified bis-β-hydroxyethyl terephthalate. The analytical valuesof the quality of the obtained purified bis-β-hydroxyethyl terephthalateare shown in Table 1.

TABLE 1  1. optical density 0.059  2. acid value (KOH mg/g) 0.4  3.saponification value (KOH mg/g) 439  4. melting point (° C.) 112  5.whiteness L = 98.7, a = −0.7, b = 1.2  6. total weight of cations (ppm)0.76  7. total weight of anions (ppm) 0  8. bis-β-hydroxyethylterephthalate (wt %) 97.93  9. mono-β-hydroxyethyl terephthalate (wt %)1.33 10. oligomer (wt %) 0.74

The optical density in Table 1 is used to evaluate quality ofbis-β-hydroxyethyl terephthalate and proportional to the content of acolored product. It was obtained by measuring the absorbance of a 10%methanol solution at a wavelength of 380 mμ and a cell length of 10 mm.The whiteness was measured by a color/color difference meter andexpressed by L (brightness), a (redness) and b (yellowness) values of aHunter method.

(2) 5 kg of the obtained purified bis-β-hydroxyethyl terephthalate in apowder form at normal temperature and 1 g of tributyl phosphate in aliquid form at normal temperature were charged into a 10 liter autoclaveequipped with a stirrer and vacuum generator, the inside of theautoclave was fully substituted with nitrogen gas, and thebis-β-hydroxyethyl terephthalate was molten by heating at 130° C. undera nitrogen gas atmosphere. Thereafter, a dispersion obtained bydispersing 0.6 g of germanium dioxide in 275 g of ethylene glycol atnormal temperature was added as a polymerization catalyst under anitrogen gas atmosphere, the temperature was raised to the boiling point(197° C.) of ethylene glycol over 10 minutes under agitation, andheating and stirring were carried out at normal pressure and 197° C. for30 minutes while evaporated ethylene glycol was totally refluxed.Thereafter, this bis-β-hydroxyethyl terephthalate was heated to 245° C.over 15 minutes, and then ethylene glycol was distilled off at normalpressure and 245° C. for 30 minutes to obtain an oligomer. The averagepolymerization degree of the obtained oligomer was 3.2. The obtainedoligomer was polycondensed at 280° C. and 90 Pa (0.7 mmHg) for 3 hoursto obtain polyethylene terephthalate. The analytical values of qualityof the obtained polyethylene terephthalate are shown in Table 2. Theobtained purified bis-β-hydroxyethyl terephthalate and polyethyleneterephthalate had an extremely high quality level for practical use.

TABLE 2 1. intrinsic viscosity ([η]) 0.695 2. diethylene glycol (wt %)1.26 3. carboxyl terminal group (μeq/g) 9.9 4. whiteness L = 83.0, a =−2.2, b = −4.5

The intrinsic viscosity in Table 2 was measured in orthochlorophenol at30° C. The whiteness was measured by a color/color difference meter andexpressed by L (brightness), a (redness) and b (yellowness) values ofthe Hunter method.

Example 2

5 kg of the purified bis-β-hydroxyethyl terephthalate in a powder format normal temperature after molecular distillation obtained in Example 1and 1 g of tributyl phosphate in a liquid form at normal temperaturewere charged into a 10 liter autoclave equipped with a stirrer andvacuum generator, the inside of the autoclave was fully substituted withnitrogen gas, and the bis-β-hydroxyethyl terephthalate was molten byheating at 130° C. under a nitrogen gas atmosphere. Thereafter, adispersion prepared by dispersing 3.2 g of antimony trioxide in 800 g ofethylene glycol at normal temperature was added as a polymerizationcatalyst under a nitrogen gas atmosphere, the temperature was raised tothe boiling point (197° C.) of ethylene glycol over 10 minutes underagitation, and heating and stirring were carried out at normal pressureand 197° C. for 30 minutes while evaporated ethylene glycol was totallyrefluxed. Thereafter, this bis-β-hydroxyethyl terephthalate was heatedto 245° C. over 15 minutes, and then ethylene glycol was distilled offat normal pressure and 245° C. for 30 minutes to obtain an oligomer. Theaverage polymerization degree of the obtained oligomer was 3.5. Theobtained oligomer was polycondensed at 280° C. and 90 Pa (0.7 mmHg) for3 hours to obtain polyethylene terephthalate. The analytical values ofquality of the obtained polyethylene terephthalate are shown in Table 3.The obtained polyethylene terephthalate had an extremely high qualitylevel for practical use.

TABLE 3 1. intrinsic viscosity ([η]) 0.687 2. diethylene glycol (wt %)1.35 3. carboxyl terminal group (μeq/g) 10.2 4. whiteness L = 83.0, a =−2.2, b = −4.0

The intrinsic viscosity in Table 3 was measured in orthochlorophenol at30° C. The whiteness was measured by a color/color difference meter andrepresented by L (brightness), a (redness) and b (yellowness) values ofthe Hunter method.

Comparative Example 1

5 kg of commercially available bis-β-hydroxyethyl terephthalate (ofNisso Maruzen Chemical Co., Ltd.) (total cation content of 383.2 ppm andtotal anion content of 430 ppm) in a powder form at normal temperatureand 1 g of tributyl phosphate in a liquid form at normal temperaturewere charged into a 10 liter autoclave equipped with a stirrer andvacuum generator, the inside of the autoclave was fully substituted withnitrogen gas, and the bis-β-hydroxyethyl terephthalate was molten byheating at 130° C. under a nitrogen gas atmosphere. After 0.6 g ofpowdery germanium dioxide was added as a polymerization catalyst under anitrogen gas atmosphere, the temperature was raised to 240° C. over 20minutes under agitation, and polyethylene terephthalate was obtained bypolycondensation at 280° C. and 90 Pa (0.7 mmHg) over 3 hours. Theanalytical value of quality of the obtained polyethylene terephthalateare shown in Table 4. The obtained polyethylene terephthalate wascolored yellow brown to a visible extent though its viscosity increased.

TABLE 4 1. intrinsic viscosity ([η]) 0.693 2. diethylene glycol (wt %)2.24 3. carboxyl terminal group (μeq/g) 16.3 4. whiteness L = 87.0, a =−1.9, b = 3.8

The intrinsic viscosity in Table 4 was measured in orthochlorophenol at30° C. The whiteness was measured by a color/color difference meter andrepresented by L (brightness), a (redness) and b (yellowness) values ofthe Hunter method.

What is claimed is:
 1. A method for producing a polyester, whichcomprises (1) providing polyester production raw materials includingbis-β-hydroxyethyl terephthalate containing cations and anions in atotal weight of not more than 50 ppm, ethylene glycol and at least onepolymerization catalyst selected from the group consisting of antimonytrioxide and germanium dioxide; (2) heating the polyester production rawmaterials without substantially distilling off ethylene glycol; and (3)polycondensing the raw materials by heating under reduced pressure whiledistilling off ethylene glycol.
 2. The method of claim 1, wherein thepolyester production raw materials further include at least one memberselected from the group consisting of terephthalic acid, isophthalicacid and cyclohexanedimethanol.
 3. The method of claim 2, wherein thepolyester production raw materials further include terephthalic acid,and the content of terephthalic acid is 0.05 to 20 mols based on 1 molof the bis-β-hydroxyethyl terephthalate containing cations and anions ina total weight of not more than 50 ppm.
 4. The method of claim 2,wherein the polyester production raw materials further includeisophthalic acid and/or cyclohexanedimethanol, and the content of theisophthalic acid and/or cyclohexanedimethanol is 0.05 to 50 mols basedon 1 mol of the bis-β-hydroxyethyl terephthalate containing cations andanions in a total weight of not more than 50 ppm.
 5. The method of claim1, wherein polycondensation is carried out in the presence of aphosphorus-based stabilizer.